tailieunhanh - Clutches and brakes design and selection P7

All three of these brake or clutch types have no wearing parts because the torque is developed from electromagnetic reactions rather than mechanical friction. Electronic controls and a rectifier to provide direct current are required, however, for their operation. They are, nevertheless, not usually referred to as electric brakes because that term had been reserved earlier to denote friction brakes which are electromagnetically activated: those in which an electric current through a coil induces a magnetic field that engages a shoe and drum, as pictured in Chapter 4. . | 7 Magnetic Particle Hysteresis and Eddy-Current Brakes and Clutches All three of these brake or clutch types have no wearing parts because the torque is developed from electromagnetic reactions rather than mechanical friction. Electronic controls and a rectifier to provide direct current are required however for their operation. They are nevertheless not usually referred to as electric brakes because that term had been reserved earlier to denote friction brakes which are electromagnetically activated those in which an electric current through a coil induces a magnetic field that engages a shoe and drum as pictured in Chapter 4. Because particular construction variations from manufacturer to man-facturer can have a strong effect on the performance characteristics of these brakes in terms of magnetic fringing and local variation of the electric fields we limit our discussion of the theoretical background of these brakes to the underlying equations only. This is consistent with the design practices associated with these brakes. They are often designed in the laboratory by a combination of theory and trial and error because our present theory is not adequate to handle small geometric effects on the electric and magnetic fields between conductors that are very close to one another. Incidentally these theoretical shortcomings are also evident in present-day design procedures for high-frequency antennas. Copyright 2004 Marcel Dekker Inc. Since these formulas are not presented with sufficient detail for the reader to design magnetic particle hysteresis or eddy-current brakes they will not be summarized at the end of the chapter. I. THEORETICAL BACKGROUND The basic equations that define the theory used in explaining the generation of eddy currents and of hysteresis loops are presented in the remainder of this section. A more complete discussion of the theory beginning with Maxwell s equations equations 1-1 along with the derivation of the subsequent relations may be found .